Liquid crystal compositions are used in conventional passive mode liquid crystal elements such as 90.degree. twisted nematic mode, guest/host mode, supertwisted mode and SBE mode liquid crystal elements and in active matrix mode liquid crystal elements, for example, a nonlinear two-terminal element such as a diode and a three-terminal element such as a thin film transistor (hereinafter referred to as "TFT" on occasion) as switching elements, and each of these liquid crystal compositions is a mixture which is used so as to cause a compound having a CN group on a terminal or a side chain to exhibit a positive or a negative dielectric anisotropy (hereinafter referred to as ".DELTA..epsilon." on occasion). In this connection, when induced by the CN group on the terminal, the dielectric anisotropy is largely positive; and when induced by the CN group on the lateral, chain the dielectric anisotropy is largely negative. For example, in the case of the 90.degree. twisted nematic mode liquid crystal display element which occupies most of the commercially available liquid crystal elements, a threshold voltage V.sub.c regarding voltage-capacitance characteristics can be represented by the following formula containing a dielectric anisotropy: .DELTA..epsilon., and Frank's elastic constants, K.sub.11, K.sub.22 and K.sub.33 : ##EQU1##
Therefore, it is effective in decreasing the threshold voltage V.sub.c that the liquid crystal element contain a compound having a positive and large .DELTA..epsilon., i.e., a compound having the CN group at a terminal as a constitutional component of the composition. In addition, the value of .DELTA..epsilon. has an influence on the threshold voltage V.sub.c. That is, the larger the .vertline..DELTA..epsilon..vertline. is, the smaller the value of the threshold voltage V.sub.c is. In consequence, the conventional liquid crystal composition permits driving the liquid crystal display element at a low voltage, e.g., several volts by adjusting the content of the compound having the CN group at the terminal or the lateral in the above-mentioned system, which is the greatest feature of the liquid crystal display element.
In recent years, the enlargement of the application range of the liquid crystal display elements has led to the requirement of the liquid crystal composition of the passive mode displays, active matrix mode displays and the like having a high reliability of a small electric current consumption, a high specific resistance and the like as well as the requirement of the liquid crystal element having a high display contrast.
However, a group having a strong polarity such as the CN group contributes to the occurrence of the above-mentioned dielectric anisotropy, but it is undesirable in the electric current consumption, the specific resistance and the display contrast. This reason is not elucidated apparently even by a person skilled in the art, but the present inventors presume that the CN group at the terminal or on the lateral chain would interact somewhat with ionic impurities present in the display element and have a bad influence on the electric current consumption, the specific resistance and the display contrast. Drawbacks of a compound having a CN group are as follows: In the case of the passive mode element, the reliability deteriorates. That is, the electric current consumption increases and specific resistance lowers, and with regard to the display characteristics, irregularity in the contrast of the display and decline of the contrast tend to take place Furthermore, in the case of the active display element, the deterioration of the reliability is more noticeable than in the case of the passive display element under the influence of a drive current of a two-terminal or three-terminal switching element, which leads to an increase of the electric current consumption and the decline in the specific resistance. In particular, this decline in the specific resistance further brings about a secondary bad influence such as the deterioration of contrast of the active matrix displays.
For example, the contrast of the TFT display element shown in FIG. 1 is closely connected with its signal voltage holding ratio. The signal voltage holding ratio denotes the degree of the drop of the signal voltage applied to a TFT picture element containing the liquid crystal in a prescribed frame cycle. Therefore, when a drop of the signal voltage is not present, deterioration of the contrast does not occur. Furthermore, the signal voltage holding ratio synergistically deteriorates, as the capacitance of a capacitor (C.sub.s) disposed in parallel with the liquid crystal and the specific resistance of the liquid crystal (LC) decrease. In particular, when the specific resistance of the liquid crystal falls below a certain lower limit, the voltage holding ratio deteriorates exponentially, which case leads to an extreme decline in the contrast. Particularly when any storage capacitance (C.sub.s) is not added, on account of the simplification of a TFT manufacturing process or the like, a liquid crystal composition having a fairly high specific resistance is necessary, because a contribution of the storage capacitance cannot be expected.
As discussed above, in order to ensure high reliability and high contrast in the passive and the active mode liquid crystal elements, a liquid crystal composition involving a small current consumption and having a high specific resistance is required.
An object of the present invention is to provide a liquid crystal composition involving a small current consumption and having a high specific resistance as described above, i.e., having a high reliability, and another object of the present invention is to provide a liquid crystal display device using the above-mentioned liquid crystal composition and having high reliability and high display contrast.